The present invention relates to optical devices and more particularly to the manufacture of high quality micro-optical devices.
FIG. 1 depicts section of an exemplary conventional micro-optical element 100. The element 100 is in the form of a substantially flat glass plate 110. The glass plate 110 has a primary upper surface 110A and primary lower surface 110B. The glass plate 110 could be in the shape of a square, rectangle, circle or have some other shape. The glass plate could, for example, be for use in one or more etalons or other optical devices.
The primary upper surface 110A and primary lower surface 110B of the glass plate 110 are separated by a distance xe2x80x9cdxe2x80x9d. The transmission characteristics of the glass plate 110 between the primary upper surface 110A and primary lower surface 110B, and more particularly what is commonly referred to as the optical thickness or optical path distance (OPD), will vary with variations in the distance d. In many applications, it is important that the OPD throughout the glass plate 110 be uniform.
A single optical element, such as the plate 110, may form what is commonly characterized as a flat, from which multiple smaller optical sub-elements will be formed. More particularly, a flat is sliced to separate the multiple optical sub-elements. Each separate sub-element may, for example, then be used individually in the manufacture an optical device. Accordingly, it is desirable for each of the multiple sub-elements within the flat, e.g. glass plate 110, to have uniform transmission characteristics, which in turn requires that that the OPD throughout the flat be uniform or uniform within an acceptable tolerance.
To establish a uniform OPD throughout the sub-elements, the conventional practice is to first physically measure the thickness of the optical element. Polishing is performed to remove material from one or more surface of the element based on the physical measurement. The uniformity of the OPD for the element is then optically measured. If required, further polishing and optical measurement of element""s OPD uniformity is performed. Once sufficient uniformity has been achieved, the element is sliced in order to separate the individual optical sub-elements.
For example, referring again to FIG. 1, in the case of the glass plate 110 the distance between the primary upper and lower surfaces 110A and 110B at various locations on the plate is measured, using a micrometer, to determine the distance d at each of the measured locations. These determined distances are then used to determine, on a relatively coarse basis, the uniformity of the apparent OPD, which can be computed using the determined distances as is well understood in the art.
Based on the measured distances, a determination is also made as to how much material must be removed from particular areas of one or both of the primary surfaces 110A and 110B to form a plate having a sufficiently uniform thickness d, and therefore a sufficiently uniform apparent OPD, for the particular application. The material is removed by polishing the optical element.
The goal of this initial polishing is to make the surfaces 110A and 110B of the plate perfectly parallel, and hence the apparent OPD perfectly uniform. However, as is recognized by those skilled in the art, the true OPD and the apparent OPD will often vary for at least two reasons. First, the true distance d may differ from the physically measured distance. This is particularly true in the fabrication of micro-optical elements. Additionally, there may be variations in the refractive index of the glass forming the optical element, e.g. the plate 110. Hence, even if the plate 110 has the exact same thickness, and therefore the exact same apparent OPD, at two locations, the true OPD at these locations may vary due to differences in the refractive indexes of the plate material at these locations. Accordingly, to obtain uniformity of the true OPD, it may be necessary for the thickness of the element, e.g. the distance d, to vary at different locations. Therefore, even if the initial polishing results in the surfaces 110A and 110B of the plate being perfectly parallel, this may not result in sufficient uniformity of the true OPD.
Therefore, after the initial polishing, an interferometer (not shown) is typically used to perform an optical measurement by directing a broad beam of light 120 over the entire surface 110A of the plate 110. As is well understood in the art, by visually examining the shading of the light 130 passing through the plate 110, a more accurate determination of the uniformity of the true OPD can be determined.
If the plate surfaces 110A and 110B are perfectly parallel, making the distance d perfectly uniform, and the plate material has a constant refractive index throughout the plate, the OPD will also be perfectly uniform. In such a case, the interferometer light 130 which passes through the plate will appear either light or dark, but in any event of a constant shade of gray or of a constant color when viewed with the naked eye. It will be recognized that the fact that the passing light 130 is light or dark is unimportant. Rather, what is important is that the passing light 130 appears to have a uniform shade or color.
However, if the passing light 130 appears to have a non-uniform shade or color, further polishing is performed on those areas of the appropriate surface(s) 110A and 110B corresponding to the areas of non-uniform shading. This further polishing will be performed whether the non-uniform shading or color, reflecting a non-uniformity of the true OPD, is caused by the plate surfaces 110A and 110B not being perfectly parallel or by the plate material having a varying refractive index or both. It will be understood, if the refractive index varies, uniform shading, and hence a uniform true OPD, can only be achieved if the distance d actually varies slightly in different areas of the optical element, to offset the index variations. After this further polishing, the plate 110 is re-checked, using the interferometer. After inspection indicates a sufficiently uniform true OPD, the plate 110 is sliced into multiple optical sub-elements.
Hence, if the fabricator perceives, through a visual inspection, that a variation in shading or color exist in the middle of the optical element, a small portion of the material in the middle of the element is removed by polishing. If visual inspection of the passing light indicates that the shading is sufficiently uniform, no further polishing is performed and the optical element can be sliced into respective sub-elements or used in a further flat assembly as will be described below with reference to FIG. 2.
FIG. 2 shows an exemplary portion of a conventional flat of micro-optical devices, commonly characterized as etalons. The depicted etalon is formed of a portion of top plate 210A and bottom plate 210B, which are both formed of glass. Plates 210A and 210B are substantially identical to plate 110 of FIG. 1. The glass plates 210A and 210B are separated by spacers 260A and 260B to create a cavity 270. It will be understood that additional spacers (not shown) are used to separate the plates 210A and 210B to form the other etalons which will be sliced from the flat.
In the FIG. 2 example, since air fills the cavity 270, the depicted etalon is of the type commonly referred to as an air space etalon. As will be recognized by those skilled in the art, if glass filled the cavity 270, the etalon would be of a type commonly referred to as a solid etalon. If water, or some other liquid, filled the cavity 270, the etalon would be of a type commonly referred to as a liquid etalon.
Also disposed within the cavity is a top coating layer 230A formed on lower main surface of the top glass plate 210A, and a bottom coating layer 230B formed on the upper main surface of the bottom glass plate 210B. The top and bottom coatings are separated by a distance xe2x80x9cdxe2x80x9d, which can be used to determine the apparent optical thickness or OPD of the cavity 270. The transmission characteristics and hence the OPD of the etalon will vary with variations in the OPD of the glass plate 210A, glass plate 210B and cavity 270.
In the case of the cavity 270, conventionally the distance d is approximated by physically measuring the spacers 260A and 260B, which form the cavity 270, at various locations. This approximation is used to determine the uniformity of the apparent OPD of cavity 270. If deemed necessary, the spacers 160A and/or 160B are polished to modify d, and hence the uniformity of the apparent OPD of the cavity 270.
After any required polishing of the plates 210A and 210B, as discussed above with reference to FIG. 1, and the spacers, including spacers 160A and 160B, which separate the plates 210A and 210B to form the individual etalons to be sliced from the flat 200, uniformity of the OPD of the flat of etalons is checked using an interferometer, in a manner similar to that described above with respect to plate 110 of FIG. 1. The upper surface, i.e. the outside surface, of plate 210A and/or the lower surface, i.e. the outside surface, of plate 210B are further polished as deemed necessary in view of the shading of the light passing through the etalon during initial and any subsequent interferometer testing, until sufficient uniformity of the true OPD is achieved. Once polishing is completed the flat assembly is sliced into multiple etalons.
There are fundamental problems with the above described conventional optical device fabrication technique, particularly when used in micro-optic applications. One problem is that the uniformity of the shading or color is determined subjectively. Hence, different inspectors may come to different determinations with respect to the uniformity of the OPD.
Further, it is often difficult for fabricators to judge the variation in that shading to the degree necessary to fabricate plates or optical assemblies having a uniform true OPD to the accuracy level required for micro optics applications. Such applications may require uniformity in the distance d of, for example, 5 nano-meters or less. Such minor distance variations are undetectable by visual inspection. Typically, distance variation of about 60 nanometers or less are not detectable using conventional techniques.
Additionally, because optical elements and devices are commonly fabricated in flats, if polishing errors occur the entire flat must often be discarded. This is because it is extremely difficult, using the conventional techniques, to determine which particular optical sub-elements are affected by the errors. This in turn adds to the time and cost of fabrication.
Moreover, because of the human element involved in the polishing and inspection, conventional techniques frequently result in polishing errors. Therefore substantial waste occurs in the conventional fabrication process.
A still further problem occurs if the optical element or device is intended to be in a form requiring non-parallel surfaces, such as a wedge. In such cases, some variations in shading are required, while others will need to be eliminated through polishing. Thus, the fabricator, using a visual inspection, must be able to distinguish between those variations which are desirable and those which are undesirable. This requires significant skill and experience on the part of the fabricator, and makes the fabrication of such optical elements and devices extremely difficult.
Accordingly, a need exists for an improved technique for fabricating optical elements and devices having the desired optical characteristics, including optical elements and assemblies having a substantially uniform true OPD and individual flats having a number of such elements and devices.
It is accordingly an object of the present invention to provide an improved technique for fabricating optical elements and devices, referred to generally as optical components, having the desired optical characteristics, including optical elements and devices having a substantially uniform true OPD and individual flats having a number of such elements or devices.
It also an object of the present invention to provide a technique, particularly suitable for use in micro-optic applications, for determining the uniformity of the true OPD of an optical element or device in an objective manner.
It is another object of the present invention to provide a technique which can be used by different inspectors to make the same determinations with respect to uniformity of the OPD of optical elements or devices.
It is yet another object of the present invention to provide a technique for fabricating optical elements or devices having a highly accurate OPD, such as that required for micro optics applications.
It is additionally an object of the present invention to provide a technique which allows acceptable optical elements or devices within a flat to be utilized and only unacceptable optical elements or devices within a flat to be discarded.
A still further object of the invention, is to provide a technique which can be used to reduce the frequency of polishing errors in fabricating optical elements or devices.
It is also an object of the invention to provide a technique which simplifies the fabrication of optical elements and devices that require non-parallel surfaces, such as those in the form of a wedge.
Additional objects, advantages, novel features of the present invention will become apparent to those skilled in the art from this disclosure, including the following detailed description, as well as by practice of the invention. While the invention is described below with reference to preferred embodiment(s), it should be understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the invention as disclosed and claimed herein and with respect to which the invention could be of significant utility.
In accordance with the invention, a system is provided for determining the uniformity of an optical component. The optical component may be a single piece of glass or other transmissive optical material with parallel surfaces or could be a more complex optical component such as an etalon, wedge or spiral shaped optical component, an optical component which has a radially varying thickness or some other complex form component. The component may be coated with a dielectric coating or non-coated. The optical component could be part of a flat of optical components, which will subsequently be sliced to separate the individual optical components from each other. Notwithstanding the type or form of the optical component, the component can be viewed as having opposed primary surfaces through which light is intended to pass during its utilization.
The system includes a laser light beam emitter for emitting a light beam to impinge on one of the primary surfaces of the optical component at each of multiple points, and for modifying a characteristic, such as the wavelength or angle of incidence, of the impinging light beam at each of the multiple points so as to have multiple different values. The light beam will be a relatively narrow spot beam and typically impinge on only one point at a time. In one preferred embodiment of the invention, the emitter is in the form of a highly accurate tunable laser. However, other types of lasers could be used including multiple non-tunable lasers or multi-power lasers. A detector detects light from the impinging light beam that passes through the other primary surface of optical component at each of the multiple points, with the light beam characteristic having each of the multiple different values.
A processor, which could be part of a personal computer (PC) or other type computer programmed with the logic to perform the described functions, determines the non-uniformity of the optical component based on the detected passing light. Beneficially, the processor generates control signals to modify the characteristic, e.g. the wavelength or angle of incidence, of the emitted light beam impinging the primary surface of the optical component at each of the multiple points, so that the impinging light beam at each point has multiple different characteristic values. Preferably, the processor determines the non-uniformity of the optical component by computing a characteristic of the optical component at each of the multiple points based on the detected passing light. Advantageously, the processor also generates a contour map of the optical component based on the determined non-uniformity.
In accordance with other aspects of the invention, the emitted light beam is directed along a path. The processor generates emitter control signals to control the emission of the light beam by the laser light beam emitter as discussed above, and alignment control signals to align each of the multiple points with the path of the emitted light beam prior to the light beam being emitted to impinge on the optical component at the applicable point. A stage, such as an X-Y stage, supports and moves the optical component in accordance with the generated alignment control signals.
In accordance with further aspects of the invention, the processor also generates polish control signals based on the determined non-uniformity of the optical component. A polisher is provided to remove material from the optical component based on the generated polish control signals.
In a particularly advantageous implementation, the stage moves the optical component in accordance with the generated alignment control signals, such that the multiple points, at which the emitted light beam impinges on the one primary surface of the optical component, form a pattern of impinge points arranged in a first grid. However, each of the impinge points in the first grid is a first distance from adjacent ones of the impinge points in the grid. Based on the generated polish control signals, the polisher removes material from at least one of the primary surfaces of the optical component at multiple polish points. The polish points also-form a pattern of polish points arranged in a second grid which corresponds to the first grid. However, each of the polish points in the second grid is a second distance from adjacent ones of the polish points in the grid. The second distance may be the same as or different than the first distance. If different, the second distance will typically be smaller than the first distance.
The processor may, for example, determine the non-uniformity of the optical component by computing either an apparent optical thickness or optical path distance (OPD) of the optical component at each of the multiple points based on the detected passing light. The processor then normalizes the computed apparent optical thickness or optical path distance (OPD) of the optical component at each of the multiple points based on the detected passing light. The processor generates the polish control signals based on the normalized apparent optical thickness or optical path distance (OPD) at each of the multiple points. In such a case, the polisher will remove material from at least one of the primary surfaces of the optical component at the multiple points based on the generated polish control signals. The above described contour map could represent the computed apparent optical thickness or optical path distance (OPD) of the optical component at each of the multiple points.
According to yet another aspect of the invention, the processor determines a maximum non-uniformity with respect to all of the multiple points and compares the determined maximum non-uniformity with a threshold maximum non-uniformity value. The maximum non-uniformity is preferably determined with respect to all of the multiple points based, for example, on the maximum difference between the computed apparent optical thicknesses or optical path distances (OPDs) of the optical component, at any two of the multiple points. Based on this comparison, the processor can determine if polishing is required. If so, the polish control signals are generated. If not, the polish control signals need not be generated.
According to-still other aspects of the invention, the processor may be further configured to generate a pass indicator if it is determined that polishing is not required and a fail indicator if it is determined that polishing is required.
The polisher may be programmed to generate a predicted result of the material removal based on the polish control signals. The processor could then determine whether to proceed with the material removal based on the generated predicted result. If so, the processor will generate the polish control signals to direct the polisher to remove material from the optical component. Otherwise, the processor need not generate the polish control signals.
Beneficially, a user input device is provided for entering a minimum thickness of the material to be removed by the polisher. Based on this input and the determined non-uniformity of the optical component, the polisher generates the predicted result. If the processor determines not to proceed with the removal of material based on the generated predicted result, the user input device can be operated to enter a another minimum thickness of material to be removed by the polisher. The polisher will then generate another predicted result based on the determined non-uniformity of the optical component and the later identified minimum thickness. The processor can now determine whether to proceed with the material removal based on the later generated predicted result.
The invention can also be used to assist in predicting the effects of subsequent coating on an optical component, thereby allowing the polishing of the optical component taking into account the subsequent application of the coating after polishing. For example, after the characteristic, e.g. the apparent optical thickness or OPD of a coated optical component has been determined by comparing the apparent optical thickness or OPD determined after coating to the apparent optical thickness or OPD determined before coating, the polisher can remove material from another, or second, non-coated optical component taking into consideration the determined characteristic of the coated optical component.
More particularly, the laser light beam emitter can emit a light beam to impinge on one of the primary surfaces of the non-coated optical component at each of multiple points, and modify the applicable characteristic, e.g. the wavelength or angle of incidence, at each of the multiple points so as to have multiple different values. The detector detects light from the impinging light beam that passes through the other primary surface of non-coated optical component at each of the multiple points, with the light beam characteristic at each of the multiple different values.
The processor determines the non-uniformity of the non-coated optical component based on the detected passing light. The processor then generates the polish control signals based on the determined non-uniformity of the non-coated optical component and the determined non-uniformity of the coated optical component. Finally, the polisher removes material from the non-coated optical component based on the generated polish control signals.
It should be understood that the polisher may remove material from a coated or non-coated surface of the optical component, and the polishing may be performed on the impinge surface, the opposing surface, or both primary surfaces of the optical component at the multiple polish points. The optical component may be polished so as to have a complex shape, such as that of a wedge or spiral, or a non-complex shape. Rather than removing material, material could be added to the optical component based on the determined non-uniformity of the optical component. If the optical component forms part of a flat of multiple optical components, the optical component may be marked as unacceptable based on the determined non-uniformity. A high power laser having a beam path co-axial with that of the laser described above could be controlled by the processor to perform such marking. In any event, after measurement and polishing have been completely for all the optical components forming the flat, the flat can be sliced to separate each of the optical components from the other optical components forming the flat.